The present subject matter relates to floating offshore structures and particularly to a system for connecting two floating structures by wire rope lashing to obtain a controlled relative separation and to achieve the lashing loads to be within predefined maximum lashing tension load values.
The need for this type of lashing technology is especially applicable when implemented with a tender assist drilling unit (TADU) for an offshore production facility based drilling and or completion operation.
In many deep water drilling and production installations, a floating tender assist drilling unit (TADU) is lashed to an adjacent offshore production facility or platform to assist in the drilling and production operations. This TADU can be any type of semi submersible or barge hull form. Both the TADU and the platform are typically moored to the seabed, and they are lashed to each other so as to restrict relative movement between the two structures, thereby to facilitate the transfer of drilling consumables, supplies and personnel from one structure to the other and hook-up of control and fluid lines between the two structures. The lashing mechanism must be capable of maintaining the relative movement within predefined limits that allow normal operation throughout environmental conditions that can be expected during the course of a normal year (a “one-year environment”), and that allow limited operations, including the maintenance of drilling circulation and control, throughout worst-case conditions to be expected during a typical ten-year period (a “ten-year environment”). The one year and ten year environments are established and described by Metocean.
In a 100-year extreme weather condition the system must be capable of increasing the separation distance to a storm safe distance and at the same time function as a 100-year storm safe coupled mooring system. This is obtained by having the four off lashing lines connected to the TADU forward mooring winches for separation control during storm separation and pull back for normal operation.
The present system for coupling two bodies is either a fixed wire/rope/chain coupling lines by doing a fixed length connection between the two bodies.
The downside of this fixed length type of lashing is the lack of control over the lashing loads acting between the two floating structures. As a consequence this can add to the maximum overall horizontal acceleration on the drilling equipment set installed onto the offshore production facility, with the consequence of potentially de-rating of the drilling facility operational specifications and subsequent reduction of drilling efficiency (drilling up-time).
Additionally, this type of lashing will require a full disconnect between the two floating structures during an extreme weather event to be storm safe. This disconnect will require assistance of additional support vessels and add time for the overall extreme weather preparation window required.
An alternative solution has been to utilize a wire/nylon/wire lashing hawser for a spar based production facility. This system will require an extensive length of nylon section that can only be achieved by routing the nylon-based hawser down the spar hull. This system is described in patent publication US2007/0119359 the entirety of which is incorporated by reference.
The down side of this system is that it is specific to spar hulls. It is partly submerged by having the majority of the nylon-hawser routed down the spar hull. Additionally, this is not a system that lends itself to be retrofitted. It is not possible to adjust any stiffness characteristic for this system after installation. Also, this system will require separate fixed lashing lines to be installed during close proximity of the two floating structures during heavy lift operation.
The present disclosed subject matter addresses these shortcomings, for example, not being floating structure independent. It may be used on any type of floating structure like; semi-submersible, tension leg platform, spar, barge or mono hull. The disclosed subject matter also addresses the need for relative separation control between the two floating bodies and the lashing tension load control. It also eliminates the necessity for disconnect during extreme weather to make the structures storm safe. It also addresses the need to have an additional fixed close proximity lashing during heavy load transfer between the two floating structures.
Broadly, the present subject matter is a system for lashing a first floating structure, such as a TADU, to a second floating structure, such as a production platform, which include a set of lashing lines, each extending from an anchor point of the first floating structure through a hydraulic-pneumatic tensioner system to a fixed anchor of the second floating structure.
In a preferred embodiment each of the lashing line first or free ends are connected to a winch cable that is wound on a winch on the first structure or TADU. The lashing is preferably a wire rope connected by suitable connection means connected to an anchor wire winch of the first floating structure.
The lashing wire is reeved over upper and lower sheaves, or sheave cluster of the hydraulic-pneumatic tensioner system. The lashing wire is routed over to the second floating structure and secured to an anchor mechanism at the structure. This anchor can either be a fixed anchor or a rotating anchor for the purpose of easing slipping and cutting of the lashing wire as part of the scheduled maintenance of the lashing system.
These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.